Registration is now open for the next Mathematics Learning Trajectory (MLT) MOOC-Ed (Massive Open Online Course for Educators)! Running from October 7 through November 25, the seven-week course is designed as professional development for anyone involved in mathematics education and in implementing the K–8 Common Core Math Standards at the classroom, school, or district level. The first course, “The Equipartitioning Learning Trajectory: a K-5 Foundation for Rational Number Reasoning,” examines how equipartitioning, grounded in fair sharing activities that create equal-sized groups or parts, provides a robust foundation for students’ conceptual understanding of division, multiplication, fraction, and ratio.